The invention pertains to speed measuring devices and more particularly to a multiple coil electromagnetic pickup capable of converting into signals the flux variations produced by the passing of the teeth of a sound wheel connected to a moving mechanism.
Such pick-ups are known, comprising a magnetic bar at the end of which is wound a multilayer coil. More generally, and in order to reduce the diameter of such pick-ups, the coil is mounted on a pole piece which is mechanically attached to the extremity of a bar with as small an air gap as possible. The use of electromagnetic pickups in aviation, and more generally in circuits which must have particularly high reliability, has led technicians to duplicate each device, resulting in more significant bulk. The problem is appreciably the same when it is necessary to reproduce the received information on a second reading or recording device. This requirement has led to the present technology, consisting of placing several windings on a single magnetic circuit. This technique entails certain drawbacks:
it produces a mutual inductance higher than 0.95; since the windings then behave as a transformer, every parasitic signal which arrives at one of the windings is transmitted to the others due to the inductive and capacitive effects. The result is that the operational reliability of devices connected to such windings is compromised;
when one winding is short-circuited, the output power available in the other is significantly attenuated and may affect the reliability of the system. In order to make this phenomenon acceptable, the pickup must be overdimensioned or the procedures of French Pat. No. 2,075,781 must be applied. According to that patent, the signal attenuation in the non-short-circuited coil is eliminated by magnetically coupling the two coils. Thus, when one of the coils is short-circuited, the induced voltage in the other coil increases slightly. The increase in the induced current in the short-circuited coil causes a larger inverse flux to develop in the core of the non-short-circuited coil and, consequently, an increase in the voltage supplied by the coil. A pickup embodied in this manner is a closed magnetic circuit pickup and is relatively difficult to make, given the fact that the two pole pieces form a stator, one of them being placed at one end of the magnet and the other at the end of the coils, which is not in contact with the magnet. This relatively bulky arrangement cannot yield a pickup of very small size.
Another proposal consists of winding multiple windings in accordance with the "several wires in hand" technique; but in this case the electrical insulation between the windings is weak and causes significant problems.